In the field of implantable prostheses, various porous polymeric extruded and textile materials have been used as substrates therefor. It is well known to coat the outer surfaces of such porous substrates with various compositions to enhance the biocompatibility of the prosthesis. For example, certain coatings can enhance the healing process by encouraging cellular ingrowth into the microporous structure and prevent bleeding from, e.g. suture holes made in the prosthesis, and/or deliver therapeutic agents to the site of implantation.
Examples of extruded tubular substrates which have been used as implantable prosthesis include, for example, polyurethane, polycarbonate and a fluorinated polymers such as polytetrafluoroethylene (PTFE). For example, expanded polytetrafluoroethylene (ePTFE) porous tubes are made by stretching and sintering and have been used as tubular prostheses for artificial blood vessels for a number of years. These polymeric tubes have certain advantages over conventional textile prostheses, but also have disadvantages of their own. An ePTFE tube has a microporous structure consisting of small nodes interconnected with many thin fibrils. The diameter of the fibrils, which depend on the processing conditions, can be controlled to a large degree, and the resulting flexible structure has greater versatility in many aspects than conventional textile grafts.
For example, ePTFE grafts can be used in both large diameter, i.e. 6 mm or greater artificial blood vessels, as well as in grafts having diameters of 5 mm or less. See, for example, co-owned U.S. Pat. No. 5,716,660 and U.S. Pat. No. 5,665,114 both of which are incorporated by reference herein.
One particular problem with ePTFE tubes, however, is their tendency to leak blood at suture holes and to propagate a tear line at the point of entry of the suture. As a result, numerous methods of orienting the node and fibril structure have been developed to prevent tear propagation. These processes are often complicated and require special machinery and/or materials to achieve this end.
Additionally, ePTFE arterial prostheses have been reported as suffering from poor cellular infiltration and collagen deposition of the microporous structure by surrounding tissue. Numerous attempts to achieve improved blood compatibility and tissue binding properties have thus far fallen short. For example, in a study reported by Guidoin, et al., cellular infiltration of the e-PTFE microporous structure was observed as being minimal. See "Histopathology of Expanded PTFE", Biomaterials 1993, Volume 14, No. 9.
In an attempt to produce viable endothelial cell monolayers on graft surfaces, cryopreserved cultivated human saphenous vein endothelial cells were cultivated on reinforced PTFE prostheses. Prior to seeding of the endothelial cells on the prostheses, the graft surface was precoated with human fibronectin. Such a process was reported to have discouraging results. See, Kadletz, et al. in "In vitro lining of Fibronectin Coated PTFE Grafts With Cryopreserved Saphenous Vein Endothelial Cells", Thorac. Cardiovasc. Surgeon 35 (1987) 143-147.
Grafts having a monolayer of cells disposed on an outer surface thereof, however, are ineffective because the cells are easily sloughed off the graft. In particular, seeding the surface of a graft with cells results in poorly formed basement membrane which results in the loss of a significant number of cells from the surface of the graft.
More recently a study using laminin, collagen type I/III, as well as fibronectin as a precoating material prior to seeding of endothelial cells on ePTFE grafts was performed by Kaehler et al. This study reported that cell adherence and cell spreading were distinctly superior on the surfaces which were precoated with fibronectin/type I/III collagen. See "Precoating Substrate and Surface Configuration Determine Adherence and Spreading of Seeded Endothelial Cells on Polytetrafluoroethylene Grafts", Journal of Vascular Surgery, Volume 9, No. 4 April (1989).
In addition to coating implantable prostheses with, for example, extracellular matrix proteins, such as collagen, attempts have been made to implant polymeric piezoelectric materials within the body to stimulate the healing process. In particular, when a piezoelectric material is subjected to a mechanical stress, it produces an electrical field. Such electrical fields are known to stimulate cell growth. For example, piezoelectric films and membranes have been developed for ossification enhancement and regenerating severed nerves. See, U.S. Pat. Nos. 5,298,602 and 5,030,225 respectively. Such films and membranes, however, are often difficult to manufacture and are limited in their application to the body.
Accordingly, there is a need for a porous implantable device which is capable of acting as a support for a variety of therapeutically useful substrates. In particular, it would be desirable to have a porous implantable material that can contain or be substantially filled with a fluid which solidifies and is cross-linkable to form an insoluble, biocompatible, biodegradable material, such as collagen. There is also a need for an implantable device made from a porous polymeric material which is able to accommodate a fluid which contains one or more populations of cells that can produce and secrete therapeutically useful agents into the local environment. Still further, there is a need for an implantable device made from a porous polymeric material which can accommodate a piezoelectric composition which has the ability to stimulate cell ingrowth via the generation of electrical fields in response to mechanical stress. The present invention is directed to meeting these and other needs.